Study of Type 6 Secretion System in Non-Toxigenic Strains of Vibrio cholerae O1 Biovar El Tor and Their Competitive Features

The aim of the work was to study the structure of the genes of type 6 secretion system in non-toxigenic strains of Vibrio cholerae O1 serogroup, biovar El Tor and to identify their competitive features.
Materials and methods. The studies were conducted on non-toxigenic strains isolated in the Russian Federation from patients or environment between 1981and 2017, stored in the State Collection of Pathogenic Bacteria of the Russian Anti-Plague Institute “Microbe” of the Rospotrebnadzor. For bioinformatics analysis, the BLAST v.2.15.0 and BioEdit v.7.0.9.0 algorithms were applied. The nucleotide sequences of complete genomes presented in NCBI GenBank were utilized. The competitive test was set in autoclaved river water according to the method of R. Freter et al. (1981).
Results and discussion. It has been established that the composition of type 6 secretion system clusters of in ctxA^–tcpA⁺ isolates is identical to that of toxigenic V. cholerae O1 El Tor strains and includes a large cluster and three additional – Aux-1, 2, and 3. Moreover, Aux-3 corresponds to the Aux-3^P type, characteristic of toxigenic isolates, in terms of size and gene composition. The structure of 88.9 % of the studied loci is also prototypical. The exception is three strains with a single nucleotide substitution in the gene encoding the VgrG-3 effector. A large cluster and Aux-2 have been detected in ctxAB^–tcpA^– strains. The Aux-1 cluster is absent in the genome of 22 % of the strains, and in the remaining ones only the marginal gene encoding hemolysin is present. Only two ctxAB^–tcpA^– strains contain Aux-3, which corresponds to the Aux-3E type, characteristic of aquatic vibrios. Variability in the structure of the majority of the studied genes of these strains has been revealed. When ctxAB^–tcpA^– and ctxAB^–tcpA⁺ strains were co-cultivated, almost all ctxAB^–tcpA⁺ isolates exhibited a competitive advantage. The exception was the ctxAB^–tcpA^– strain of V. cholerae M1337, which produced increased amounts of exopolysaccharide, enhancing its competitive properties. The coexistence of ctxAB^– tcpA⁺ isolates and Aeromonas hidrophila 401, which has a functional type 6 secretion system, has been demonstrated.

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